Alpha-phase silicon nitride material has been made in a variety of ways, most of them expensive, time and labor intensive. It is desirable to produce a high alpha-phase content silicon nitride from an inexpensive starting material in order to be later processed into various products. For some applications, such as refractory products, it is advantageous to convert the alpha-phase silicon nitride material to beta-phase silicon nitride material. Beta-phase material may either be in the form of densified or non-densified silicon nitride. To form densified silicon nitride material, there generally is a requirement for the inclusion of densification aids, or liquid forming agents. To form non-densified beta-phase, densifying agents must not be present in the alpha-phase silicon nitride body.
Heat treatment of the alpha silicon nitride above 1450.degree. C. can result in either densified or nondensified beta-phase material. In order for the alpha-phase material to be sintered and converted into densified beta-phase material, either densification aids or liquid forming agents must be included in the alpha-silicon nitride body before the sintering temperature is reached and maintained for a sufficiently long time to sinter the material. There are some applications, particularly as refractories, for which a non-densified beta-phase material is desirable. This material can be formed by heat treatment above 1450.degree. C., from an alpha-phase silicon nitride not containing densification aids.
Densification of silicon nitride occurs by the transformation of the alpha phase of silicon nitride into the beta phase of silicon nitride in the presence of a high temperature liquid phase, accompanied by about a 10-12 percent reduction in volume. The liquid phase promotes the conversion of the alpha-phase silicon nitride to the densified beta phase silicon nitride during sintering or densification. It has been found that densification does not generally occur without liquid forming agents. When alpha-phase material is subjected to high temperatures, conversion may be directly to beta-phase material without changes in volume, and consequently no densification.
In the past there has been a major problem associated with the processing of reaction bonded silicon nitride which is the extensive time required for preparation and nitridation of the silicon powder. Typically, in order to manufacture a reaction bonded silicon nitride, very pure silicon has been ground and mixed dry with sintering aids for long periods of time, upwards of 48 hours, and then nitrided for long times, on the order of hundreds of hours to weeks. Total fabrication times of 200 to 400 hours are not uncommon.
Reaction bonded silicon nitride is commonly prepared by reacting and nitriding the silicon (either as a powder or as a formed article) with nitrogen by exposing the silicon to a nitrogen-containing atmosphere at temperatures of to about 1420.degree. C. for times sufficient to produce the silicon nitride. It is not uncommon for the nitriding time in prior art methods to be 100-200 hours. It is normal for a small amount of nitriding aid (e.g., iron oxide or nickel oxide) to be initially mixed with the silicon powder to enhance the nitridation of the silicon during the nitriding step.
U.S. Pat. No. 4,285,895 to Mangels et al. teaches that sintered reaction bonded silicon nitride articles can be made by incorporation of a densification aid into the reaction bonded silicon nitride article, surrounding the article with a packing powder of silicon nitride and densification aid and subjecting the article and powder mixture to a temperature above 1700.degree. C. with a nitrogen atmosphere of sufficient pressure to prevent volatilization of the silicon nitride for a time sufficient to permit sintering of the reaction bonded silicon nitride articles.
Several methods for introducing the densification aid into the reaction bonded silicon nitride article are disclosed in the above referenced Mangels et al. patent. These include (1) the impregnation of the densification aid into the reaction bonded silicon nitride article; (2) incorporation of the densification aid into the cover powder and then relying upon diffusion of that densification aid into the article at the sintering temperature; and (3) incorporation of the densification aid into the silicon powder mix prior to nitriding. The densification aids cited are magnesium oxide, yttrium oxide, cerium oxide, and zirconium oxide. The Mangels et al. patent also teaches that the nitrogen pressure at the sintering temperature may be in the range of 250 to 1500 psi.
U.S. Pat. No. 4,351,787 to Martinengo et al. teaches that sintered silicon nitride articles can be prepared by forming a silicon powder mixture containing one or more sintering additives into a compact, the additives being present in the powder in an amount such as to ensure an additive content of from 0.5 to 20 wt % in the silicon nitride compact; heating the compact under a nitrogen gas blanket at a temperature not exceeding 1500.degree. C. to convert the silicon into reaction bonded silicon nitride; and sintering the reaction bonded silicon nitride compact by heating in a nitrogen gas atmosphere at a temperature of at least 1500.degree. C. Furthermore, it is taught that the silicon powder size is from 0.1 to 44 microns in size and of high purity or containing only very small amounts of nitriding catalysts. The Martinengo et al. patent teaches that any conventional sintering additive may be used. Best results are said to be achieved by using MgO, and especially in combination with Y.sub.2 O.sub.3. Other preferred additives mentioned in the patent are MgO, Y.sub.2 O.sub.3, CeO.sub.2, ZrO.sub.2, BeO, Mg.sub.3 N.sub.2, and AlN. Other examples of additives are given as Mg.sub.2 Si, MgAl.sub.2 O.sub.4, and rare earth additions such as La.sub.2 O.sub.3. Also iron can be used with advantage, usually in mixture with conventional additives such as MgO, Y.sub.2 O.sub.3, and CeO.sub.2.
As a final example of sintered reaction bonded silicon nitride practice, reference is made to U.S. Pat. No. 4,443,394 to Ezis which teaches a method for making a fully densified silicon nitride body. The basic principle taught is that silicon nitride will not sinter by itself, but requires a liquid phase at the sintering temperature. Ezis found that, by having an yttrium oxynitride and alumino-silicate liquid phase present at sintering temperatures of 1650.degree.-1750.degree. C., the need for an over pressure of nitrogen and cover or packing powder during sintering could be eliminated in order to densify the silicon nitride.
The Ezis patent teaches that, by (1) forming a nitridable mixture of: silicon powder, SiO.sub.2 (carried with the Si metal), Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 ; (2) nitriding the mixture to form a reaction bonded silicon nitride, with consequent formation of a Y.sub.10 Si.sub.6 O.sub.24 N.sub.2 phase, and an alumino-silicate which resides on the silicon nitride grains; and then (3) sintering in the 1650.degree.to 1750.degree. C. temperature range for 5-12 hours, a substantially fully densified silicon nitride is produced which exhibits a 4-point bending strength of 100,000 psi at room temperature.
The Ezis patent further teaches the need for a long ball milling time of 48 hours, preferably dry, a nitridation cycle time of 200 hours, and sintering times of 5-12 hours. Total processing time including the milling can be estimated from the preferred embodiment as approximately 260 hours.
It is, therefore, a primary object of the present invention to provide a process for making a body of non-densified beta-phase material from a high alpha-phase content starting material.